《功能陶瓷讲义》PPT课件

1、Electronic Ceramics and Their Applications,X.M. Chen (陈湘明) Department of Materials Science E-mail: Web: ,Brief Introduction of X.M. Chen,Was born in Hunan 1959 B.S. Dept. Mater. Sci. mj=average dipole moment) mj= aj E (1.3) aj - polarizability of average dipole moment; E- local electric field P=sp (

2、surface charge density) (1.4),极化机理,at= as+ao+ai+ae (1.5) ae-Electronic (Atomic) Polarization; ai -Ionic Polarization; ao-Orientation (Dipolar) Polarization; as -Space Charge or Diffusional Polarization,电位移D、电场强度E与极化强度P的关系,For case a): E=s/e0 (1.6) s - surface charge density For case b): E=(sT-sP)/e0

3、 (1.7) sT total surface charge density; sP polarazation charge density Since P= sP and sT=D (electric displacement) e0 E=P-D (1.8) D= e0 E+ P (1.9) If the dielectric is linear, P=ce e0 E, so that D= e0 E+ ce e0 E=(1+ ce) e0 E (1.10) where, ce is electric susceptibility, a tensor of the second rank,介

4、电常数(Dielectric Constant),Since D= sT, QT/A= (1+ ce) e0 U/h (1.11) QT =(1+ ce) e0 UA/h (1.12) C=QT/U= (1+ ce) e0 A/h (1.13) Since vacuum has zero susceptibility, C0=e0 A/h (1.14) If the space between the plates is filled with a dielectric of susceptibility ce, the capacitance is increased by a factor

5、 1+ ce. Permittivity e of the dielectric is defined by e =e0(1+ ce) (1.15) Dielectric constant (relative permittivity) er = e /e0=1+ ce (1.16),An individual atom or ion in a dielectric is not subjected directly to an applied field but to a local field. The internal macroscopic field Em is the result

6、ant of applied external field Ea and depolarizing field Edp, i.e. Ea-Edp. It is assumed that the solid can be regarded as comprising identifiable polarizable entities on the atomic scale. The local field EL (or Lorentz field) differ from Em since the latter is arrived at by considering the dielectri

7、c as a continuum. EL = Em+Ep+Ed (1.17) where, Ep-the contribution from the charges at the surface of the spherical cavity (imaging for the moment that the sphere of material is removed); Ed-due to the dipoles within the boundary.,Applied External Field, Internal Macroscopic Field A tail in the distr

8、ibution indicates a large distribution of relaxation time; A large range of relaxation times can indicate multiple polarization mechanisms but also losses due to conduction. A perfect or low loss dielectric would have a Cole-Cole plot that is nearly a semicircle; A poor or high loss dielectric would

9、 have a non-bounded increasing er” with increasing er.,Dielectric Strength (介电强度),Dielectric breakdown(介电击穿): All dielectrics when placed in an electric field will lose their insulating properties if the field exceeds a certain critical value.This phenomenon is called dielectric breakdown. Dielectri

10、c strength 1.48) Dielectric breakdown mechanisms Intrinsic breakdown Thermal breakdown Inonization breakdown Electrochemical breakdown,Factors Affecting Dielectric Strength,Composition: amorphous or crystalline nature, presence of mobile ions; Microstructural features: porosity, grain size, cracks,

11、flaws, secondary phases; Measurement parameters: electrode configuration, specimen thickness, temperature, time, frequency, humidity and heat transfer conditions.,Chapter 1 Elements of Dielectrics and Ceramic Insulators,II. Ceramic Insulators,Intoduction,Function of insulator in electric circuits: P

12、hysical separation of conductors and the regulation or prevention of current flow between them; Ancillary but important other functions are to provide mechanical support, heat dissipation and environmental protection for the conductors Advantages of ceramic insulators: Materials type used as insulat

13、ors: linear dielectrics Typical elements of ceramic insulator: ceramic substrates, ceramic packages,Property Requirements to Ceramic Insulators,Dielectric constant; Dielectric loss; Dielectric strength; Resistivity (1.49) Thermal conductivity; Thermal expansion coefficient; Mechanical properties.,Pr

14、operty Criteria for Good Ceramic Insulators,Dielectric constant: not greater than 30; Electric resistivity: not less than 1012 W-cm; Dielectric loss (dissipation factor): not larger than 0.001; Dielectric strength: not less than 5.0kV/mm Dielectric loss factor: not larger than 0.03,Table 1.1 Dielect

15、ric properties of Ceramic Insulators,Table 1.1 Dielectric properties of Ceramic Insulators,Table 1.2 Thermomechanical Properties of Ceramic Insulators,Table 1.2 Thermomechanical Properties of Ceramic Insulators,Materials Systems for Ceramic Insulators,Glasses Porcelains Steatite (滑石,MgO.SiO2), Forst

16、rite(镁橄榄石, 2MgO.SiO2), Cordierite(堇青石, 2MgO. 2Al2O3.5SiO2), Spinel(尖晶石, MgO. Al2O3), Mullite(莫来石, 3Al2O3.2SiO2), Single oxide and nitrides ceramics Alumina(Al2O3), Magnesia (MgO), Boron nitride (BN), Aluminum nitride (AlN), Silicon nitride (Si3N4),Property Criteria for Good Ceramic Insulators,Dielec

17、tric constant: not greater than 30; Electric resistivity: not less than 1012 W-cm; Dielectric loss (dissipation factor): not larger than 0.001; Dielectric strength: not less than 5.0kV/mm Dielectric loss factor: not larger than 0.03,Property Target for Ceramic Substrates,Low dielectric constant: e7 Lo